In general, in a battery device that is mounted on an electric vehicle (EV), a plug-in hybrid vehicle (PHEV), and a hybrid vehicle (HEV), a plurality of batteries are connected in series to form a battery unit. Here, when individual differences between batteries such as a capacitance fluctuation or a self-discharge fluctuation between batteries occurs, a fluctuation in the charged state (State of Charge: SOC) of each battery included in the battery device may occur. When this fluctuation occurs, charge control is performed based on a battery having the highest SOC among the plurality of batteries, and discharge control is performed based on a battery having the lowest SOC. Thus, the energy that can be used by the battery unit decreases. Moreover, when it is expected that the usable SOC changes over a wide range as in a PHEV or EV, deterioration of a battery is likely to progress in a high or low SOC state. Thus, it is necessary to take countermeasures to decrease the SOC when the SOC is too high or to prevent a further decrease in the SOC when the SOC is too low. Therefore, a method of eliminating an SOC fluctuation between batteries which may occur when a plurality of batteries are connected in series is proposed. In this method, a voltage equalization circuit made up of a bypass resistor and a bypass switch connected in parallel to the battery and battery control means for monitoring a battery state are mounted, and the battery control means controls the bypass switch of the equalization circuit based on a voltage fluctuation amount. That is, the method forcibly discharges a battery having a high voltage to equalize voltages.
However, when a battery having a large capacity is required, in particular, there is a limit in the equalization only during the operation of a battery device depending on the degree of a voltage fluctuation. That is, the greater the voltage fluctuation, the longer the voltage equalization. Thus, a scheme in which voltage equalization is executed even after the battery device stops operating in addition to during the operation of the battery device is discussed. As an example of such a scheme, JP-A-2002-354698 discloses a method of periodically activating the battery control means during the stop period of the battery device to control the turn on/off of the bypass switch of the voltage equalization circuit to discharge a battery having a high SOC to realize voltage equalization. Moreover, JP-A-2005-328603 discloses a method of activating the voltage equalization circuit for a predetermined period with electricity from the battery even during the stop period of the battery device to control the turn on/off of the bypass switch to discharge a discharge target battery.